Dielectric heating apparatus



June 30, v1970 T, L, WILSON ET AL 3,518,396

DIELECTRIC HEATING APPARTUS Filed May 27, 1968 2 Sheets-Sheet 1 June 30, 1970 T, L, wlLsQN ETAL DIELECTRIC HEATING APPARTUS 2 Sheets-Sheet 2 Filed May 27, 1968 www www www, www www/ www ww wwmwwwww. www www wwww wwm www @y d, w M L www A w www w ff n m WM www H Y .\l www x) .www k www ww www f\1 ww \\1 ww www www5 www www www .www %\1\| bw awww M L n m QNN 7 www ww l Nw w. \l mN m www www. www ww W w www \ww ww 11|; www

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United States Patent Office 3,518,396 Patented June 30, 1970 3,518,396 DIELECTRIC HEATING APPARATUS Thomas Lamont Wilson aud Willard H. Hickok, Louisville, Ky., assiguors to Chemetrou Corporation, Chicago, Ill., a corporation of Delaware Filed May 27, 1968, Ser. No. 732,267 Int. Cl. B23k 13/02; H05b 5/00 U.S. Cl. 219-10.53 11 Claims ABSTRACT F THE DISCLOSURE This invention relates to apparatus particularly suited fOr heating, or for simultaneously heating and pressing, a dielectric load.

Dielectric heating is accomplished by placing a dielectric substance between the two plates 0f a parallel plate capacitor, and then energizing the capacitor by connecting it to a source of radio frequency power. Heating results from hysteresis within the dielectric substance. A dielectric embossing press is a dielectric heating system in which the plates of the heating capacitor are also the platens of a hydraulic press. Dielectric embossing presses are widely used in industry for the forming and shaping of non-metallic parts.

Existing dielectric presses are large structures that are energized by a radio frequency power source operatingv at a frequency of around megacycles. In some presses, the radio frequency power is applied directly to the press platens and the press structure is made to resonate. More often, one of the platens is equipped lwith an insulated electrode to which the radio frequency energy is applied. The dielectric workload is positioned between this electrode and the other platen.

The radio frequency power source is generally a vacuum tube oscillator having a tuned plate circuit that is coupled to the press by suitable inductive or capacitive coupling means. Usually a tunable grid circuit is provided, and sometimes the press coupling means is adjustable. A variable capacitor is often included in the tuned plate circuit.

Conventional dielectric presses of the type described above must often be readjusted to compensate for frequency shifts. It has been found that differing workloads can produce shifts in the oscillator frequency of up to 40%. As a result, both the tuning of the oscillator and the coupling between the press and the power source often must be adjusted to compensate for changes in the nature of the dielectric workload. Some presses are equipped with automated grid tuning adjustment systems that respond automatically to changes in press loading. Such a system is described in U.S. Pat. No. 2,517,948. In presses that are not so automated, oscillator tuning must be readjusted manually.

Accordingly, one object of the present invention is to provide an improved dielectric embossing press that can accept differing dielectric loads without oscillator tuning readjustment, and 'without a substantial shift in oscillator frequency.

The 20 to 25 megacycle frequency used in most large industrial presses is the higest frequency that can be used in a large press structure without substantial nonuniformities arising in the press heat distribution pattern. Higher frequencies are desirable since, for a given Voltage, the amount of heat delivered to the dielectric workload increases with increasing frequency.

Thus, another object of the present invention is to provide a dielectric press that can be excited by a power source with a signal frequency of 33 megacycles, yet that can also supply heat uniformly to larger workloads than could be handled by conventional lower frequency presses.

An additional object of the present invention is to provide means fwhereby the heating pattern of a dielectric press can intentionally be made nonuniform so that workloads requiring differing amounts of heat may be processed simultaneously and so that variations in the capacity of the dielectric workload may be compensated for.

In accordance with these and many other objects an embodiment of the present invention comprises a dielectric embossing press that is able to accept, without tuning or coupling readjustment, workloads of widely varying sizes and types. This press utilizes an insulated electrode attached to one of the press platens. Energy storage circuits interconnect this electrode with the opposing platen at distributed points along the length of the press. Each energy storage circuit comprises a capacitor connected in series with the inductance of shielding material interconnecting the press platens.

The energy storage capacity of each energy storage circuit can be varied by adjustment of the associated variable capacitor. Use of these energy storage circuits insures that heating within the press can be made uniform. These energy storage circuits also make it possible for nonuniformities to be intentionally introduced into the press heat distribution pattern, thereby allowing some sections of the dielectric workload to be intentionally supplied with more heat than other sections.

Radio frequency heating energy for this press is generated by a vacuum tube oscillator circuit including a very high Q, low impedance tank circuit. 'This tank circuit comprises ian inductor constructed from two conductors, preferably coaxial cylindrical conductors, that are connected together by a large capacitor at one end and .by a short circuit at the other end. The high Q of this tank circuit stabilizes the press excitation frequency, and the low impedance facilitates its ability to accept large quantities of energy from the vacuum tube plate circuit. This tank circuit is able to store in the neighborhood of times more energy than the press can dissipate per cycle in the dielectric load even at maximum power output levels, and thus can have a minimum Q of roughly 100 and a maximum Q of about 1000 or more. This tank circuit determines and stabilizes the frequency at which the press operates.

-Energy is transferred from this tank circuit to the most centrally located of the press energy storage circuits by means of a tight, low impedance coupling arrangement. This coupling arrangement comprises a short section of one conductor within the tank circuit that is connected in series with the energy storage circuit variable capacitor. By functioning as a voltage step-down autotransformer, this coupling arrangement prevents the relatively low Q and varying impedance of the press and its dielectric load from unduly reducing the Q of the tank circuit or from significantly shifting the resonant frequency of the tank circuit.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof will best be understood by reference to the followl 3 ing specification taken in connection with the accompanying drawings, in which:

FIG. 1 is a partially diagrammatic, partly sectional perspective view of a dielectric embossing press embodying the present invention.

FIG. 2 is a sectional view of a variable capacitor suitable for use in the press shown in FIG. 1.

FIG. 3 is a fragmentary elevational view in partial section of an embossing press upper electrode assembly designed in accordance with the present invention.

Referring now to FIG. 1, a dielectric embossing press incorporating the present invention is indicated generally at 20. This dielectric embossing press comprises two sections, each constructed of a conducting material: a lower section indicated generally at 22, and an upper sectlon indicated generally at 24. The lower section 22 is attached to a hydraulic lift 26. This hydraulic lift 26 is arranged to press the lower section 22 into engagement with the upper section 24 to form a closed chamber having electrically conducting walls. A tongue 27A in a side wall 27 of the lower press section 22 is inserted into a groove 28A in a side wall 28 of the upper section 24 as the press sections 22 and 24 are moved together so that the resulting inner chamber is electrically sealed even when the two halves of the chamber are slightly out of engagement.

This tongue and groove arrangement is not critical to the present invention, and other equivalent means may be used to accomplish this result. Other means may also be used for forcing the two press sections together. Several such arrangements are disclosed in U.S. Pat. No. 2,783,- 344", for example.

Two heating electrodes 30 and 32 are mounted respectively within the two press sections 22 and 24. The lower electrode 30 is connected electrically to the lower press section 22; the upper electrode 32 is separated from the upper press section 24 by a mass of insulating material (not shown in FIG. l). Three variable capacitors 34, 36 and 38 electrically connect the upper electrode 32 to the upper press section 24 which is also formed of electrically conductive material. These three capacitors, together with the capacitance between the electrodes 30 and 32 and with the inductance of the capacitors and of the adjoining press side walls, form three inductively coupled energy storage circuit. In FIG. 1, the inductances of the capacitors and of the sidewalls are represented schematically as lumped inductances connected in series with the three capacitors 34, 36, and 38.

Directly above the upper press section 24 there is mounted a high Q, low impedance tank circuit indicated generally at 40. This tank circuit 40 is attached to the upper press section 24 by a circular duct 42. The circular duct l42 connects the interior of the tank circuit 40 to the interior of the upper press section 24. The variable capacitor 36 also connects the interior of the tank circuit 40 to the interior of the upper press section 24.

Radio frequency power is generated by a triode oscillator circuit indicated generally at 44. This circuit includes the tank circuit 40; a triode 46; a high voltage direct current source 45; and circuitry interconnecting these elements. An inductor 48 connects a triode plate lead 47 to the positive side of the source 4S. The negative side of the source 45 connects to one triode filament lead 57 or 58. A tunable series circuit including an inductor 52 and a variable capacitor connects a triode grid lead 56 to the -ground cathode terminal 57. An inductor 51, and a resistor 55 bypassed by a capacitor 53, provide a direct current path from the triode grid 56 to the negative side of the source 45. A source of lament heater power (not shown) is applied to the two triode filament leads 57 and 58.

The triode plate lead 47 is connected to the tank circuit 40 by a capacitor 54, and one triode lament lead 57 or 58 is connected to the tank circuit 40 by a series resonant circuit comprising an inductor '59 and a variable capacitor 4 '70. This series resonant circuit appears to be redundant, since it connects two points that both appear to be grounded in FIG. 1. It has been found, however, that the irnpedance of this ground connection could not be neglected at 33 megacycles, and that the series resonant circuit was desirable.

The tank circuit 40 determines the frequency at which the triode oscillator circuit 44 operates. The capacitor 50 serves as a grid tuning adjustment. Oscillation sustaining feedback signals flow from the plate 47 to the grid 56 of the triode 46 through the grid-to-plate interelectrode capacitance of the triode 46. The particular details of the oscillator circuit 44, excepting the details of the tank circuit 40, are not critical to the present invention. Any equivalent oscillator circuitry may be substituted.

The triode 46 pumps energy into and maintains a high energy level within the tank circuit 40. The capacitor 36 draws upon the energy stored within the tank circuit 40 and uses it to develop and maintain a voltage between the electrodes 30 and 32, and to maintain a current flow within the electrode surfaces. This current distributes itself throughout the press, the exact distribution depending upon the locations of lumped capacities within a particular embossing die as well as upon the inductance of the platen surfaces themselves.

The three energy storage circuits mentioned above are used to control the magnitude of the voltage at three locations along the length of the press. These three energy storage circuits are jointly responsible for the uniformity of the resulting voltage and current distributions, and they may either be adjusted to give maximum heating uniformity or they may be intentionally misadjusted to give a non uniform heat distribution. Adjustment of the energy circuits is accomplished by adjusting the three variable capacitors 34, 36, and 38.

When these three energy storage circuits are adjusted to give a uniform heat distribution pattern, this press is capable of producing a substantially larger, more uniform current distribution than can be produced in `a conventional press, even though the conventional press is operated at a lower frequency. In one test, the press 20 was able to handle ten standard test dies, while a conventional press operating at a 40% lower frequency was able to accept only six of the same test dies. When these energy storage circuits are adjusted to give a nonuniform heat distribution pattern, this press can accept loads requiring larger currents in some areas and smaller currents in other areas. For example, the door panel and rear quarter panel of an automobile can be embossed simultaneously when the energy storage circuits are adjusted to compensate for the small size and reduced capacity of the small quarter panel.

The energizing frequency of the press 20 is approximately 33 megacycles, and does not shift by more than 2% with changes in the capacity of the workloads, or by more than 6% with changes in the settings of capacitors 34, 36, and 38. By way of contrast, a conventional press equipped with a 25 megacycle excitation source may suffer a frequency drop of up to 8 megacycles or more when used to heat a large capacity load. This drop in frequency is undesirable, since lower frequencies cannot transfer heat to the dielectric workload as rapidly as can higher frequencies. The increased frequency stability of the press 20 means that both large and small capacity loads will heat at roughly the same rapid rate.

The tank circuit 40 comprises Ian inductor connected in parallel with a large capacitor 68. The inductor is formed from a pair of coaxial cylindrical conductors 60 and 61. The conductors 60 and 61 are short circuited at one end by an annular disk 62, and are respectively and individually terminated at the other end by two circular disks 66 and 64. The capacitor `68 is connected between the two disks 66 and i64. The cylindrical conductor 60 is larger than the cylindrical conductor 61 both in length and in diameter, and completely encloses the other elements of the tank circuit 40, to form a compact, selfshielding structure. The inside of the cylindrical conductor 61 is left hollow to provide a path for cooling lines traveling to the capacitor 68, as is shown in FIG. 3. The tank circuit 40 is resonant at approximately 33 megacycles when the capacitance of the capacitor 68 is 900 picofarads. To innsure the highest possible Q, all internal surfaces of the tank circuit 40 are silver plated to a minimum thickness of .0004 inch.

Energy from the triode plate 47 is fed to the disc 64 by means of the coupling capacitor 54, and to the disk 66 by the capacitor 70. Since this disc 64 is entirely enclosed within the tank circuit 40, the connecting lead 69 from the capacitor 54 must pass through a hole 71 in the conductor 60 and into the interior of the tank circuit 40.

The capacitor 36, which transfers energy from the tank circuit 40 into the press, connects to the conductor 61 at a point 72 roughly midway between the disks 62 and 64. This effectively connects a section of the conductor 61 in series with both the capacitor 36 and the inductance of the duct 42. The location of the point 72 is close enough to the disk 62 to insure a lower impedance between the point 72 and ground, and to prevent the dielectric workload from adversely lowering the Q of the tank circuit 40. The location of the point 72 is close enough to the disk 64 to insure that sufficient energy is transferred out of the tank circuit 40 and into the press to handle dielectric workloads having a high power factor.

As a direct result of the above tank circuit design, the embossing press 20 is able to accept a wide variety of differing loads without any substantial shift in operating frequency. It is therefore only rarely that oscillator tuning adjustments need be made when the dielectric workload is changed. It is, however, advisable to reduce the magnitude of the voltage supplied by the source 45 when small size loads are run to prevent overheating or too rapid heating of the workload.

Referring now to FIG. 2 there is shown a variable capacitor 100 of a type suitable to serve as the capacitor 34, 36, or 38. This capacitor can handle the high voltages and high currents encountered within a dielectric embossing press. Fixed capacitors of this same general type can also satisfactorally serve as the capacitors 54, 68 and 70. Capacitors of this type are commercially available, and may be purchased from the Jennings Radio Manufacturing Corporation, a subsidiary of International Telephone and Telegraph Corporation.

The capacitor 100 shown in FIG. 2 comprises two sets of metal plates 102 and 104 which are movable with respect to each other. One set of plates 102 is attached to the bottom 103 of a vacuum chamber 106, and the second set of movable plates 104 is suspended within the same vacuum chamber 106 by means of an adjustable metal shaft 108. The vacuum within the chamber 106 is protected by a bellows 110 which keeps out air that otherwise would leak in through a gap 112 on the side of the movable shaft 108. A cooling medium is forced through two separate capacitor cooling chamlbers 114 and 116 and into the bellows 110 by means of coolant lines or fittings 118, 120, 122 and 124, that connect to a pump (not shown). The variable capacitor is adjusted by rotating a shaft 107 which raises and lowers shaft 108.

FIG. 3 is a fragmentary sectional view showing the upper heating electrode and platen of a dielectric embossing press and it illustrates how the inventive concepts shown schematically in FIG. 1 can be incorporated into -a practical embodiment. The upper press section 24 of FIG. l is replaced by a conductive metal platen shield 226 that is reinforced by a series of heavy platen structural members 230 through 236 which are rigidly attached to the embossing press base (not shown in FIG. 3). The upper press heating electrode 32 is suspended from the metal platen shield 226 by a series of insulated hangers, two of which 239 and 240 are shown in FIG. 3. A series of vertical cylindrical insulators, all of which have been given the number 242, are attached to the upper surface of upper press heating electrode 32. These vertical insulators keep the upper press heating electrode 32 spaced a proper distance from the conductive metal platen shield 226. When the hydraulic press mechanism (not shown in FIG. 3) is actuated the vertical stresses applied to the electrode 32 are transferred to the heavy platen structural members 230, 232, 234, and 236 by the vertical insulators 242. The insulators 242 can be constructed from aluminum oxide, or any other suitable material.

The three variable capacitors 34, 36 and 38 are attached directly to the upper surface of the upper press heating electrode 32. The capacitor 36 is mounted within the circular duct 42. Since the capacitors 34 and 38 are too bulky to lit within the narrow space between the upper press heating electrode 32 and the conductive metal platen shield 226, cylindrical cavities 222 and 224 are formed in the conductive shield 226 at the locations where these two capacitors are mounted. As a result, the capacitors 34 and 38 are entirely contained within the conductive metal platen shield 226.

The upper ends of the two capacitors 34 and 38 are both connected directly to the conductive metal platen shield 226. The upper end of the capacitor 36 is connected to the point 72 on the surface of the conductor 61 by a flexible conductive connector 238. The conductor 61 thus acts as a very low impedance radio frequency voltage source connected between the upper end of capacitor 36 and the conductive metal platen shield 226. The inductive impedance at a frequency of 33 megacycles between the point 72 and the adjacent circular duct 42 amounts to only two or three ohms.

Water cooling is applied to the capacitors 34, 36, 38 and 68 to carry off heat produced within these capacitors. The press heating electrode is also water cooled, although the press electrode cooling arrangement is not shown in FIG. 3. Water cooling lines which are indicated as 200 connect the fittings 118, i, 122, and 124 on the capacitors 34, 36 and 38 to provide one or more series cooling circuits, and these same lines also connect a group of fittings 126, 128, 130, and 132 on the fixed capacitor 68. The fixed capacitor 68 is actually two identical capacitors, each having a capacity of 450 picofarads, connected in parallel. The second of these capacitors does not show in FIG. 3 because it is right behind the first capacitor. The inductance of the inductive part of the tank circuit 40* is about 0.023 microhenry, and the resonant frequency of the tank circuit 40 when connected to the capacitor 68 is about 33 megacycles.

The capacitors 34, 36, and 38 each have capacities that may be varied from 20 to 450 picofarads, In this particular embodiment mechanical stops are included to further limit the tuning range of these capacitors. The center capacitor 36 is limited to between approximately 200 and 450 picofarads, and the end capacitors 34 and 38 are limited to between approximately 20 and 300 picofarads.

By varying the value of capacitor 36 the press operator can vary the total amount of heat reaching the dielectric workload. Capacitors 34 and 38 can then be adjusted to give a uniform distribution of energy along the length of the press, or they may be adjusted to give a non-uniform energy distribution along the length of the press. As an additional aid to the press operator in adjusting these capacitors, voltmeters may be attached between the press heating electrodes to give an indication of platen voltages at various locations. For example, two voltmeters may be connected to opposite ends of the press to give an indication of the uniformity or nonuniformity of the heat distribution pattern.

While there has been described what is at present believed to -be the preferred embodiment of the invention, it will be understood that various modifications may be made therein which are within the true spirit and scope of the invention as defined in the appended claims.

What is claimed as new and is desired to be secured by Letters Patent of the United States is:

1. A dielectric heating system for heating a dielectric workload disposed between rst and second heating electrodes comprising:

a conductive member surrounding the lirst heating electrode and connecting to the second heating electrode;

a plurality of variable reactance circuit means interconnecting the outer portion of the iirst heating electrode and the conductive member, said variable reactance circuit means being spaced far enough from one another to form a plurality of inductively coupled energy storage reservoirs the energy storage capacities of which can be varied;

a source of radio frequency energy; and

energy transfer means for transferring energy from said source into one of said plurality of inductively coupled energy storage reservoirs.

2. The dielectric heating system set forth in claim 1 in which the plurality of variable reactance circuit means each includes a variable capacitor.

3. The dielectric heating system set forth in claim 1 including a conductive shield surrounding said heating electrodes,

and in which said plurality of variable reactance circuit means each includes a portion of said conductive shield 4. The dielectric heating system set forth in claim 1 including a conductive shield surrounding said heating electrodes, said conductive shield being electrically connected to one of said heating electrodes,

and in which said plurality of variable reactance circuit means each include a variable capacitor that connects the other of said heating electrodes to said conductive shield.

5.v A dielectric heating system for heating a dielectric workload disposed between heating electrodes comprising a source of radio frequency energy,

a high Q, low impedance tank circuit comprising two conductors interconnected at one end by a short circuit and at the other end by a capacitor,

a capatitive means connecting said source of radio frequency energy to said tank circuit,

a conductive shield surrounding said heating electrodes and electrically connected to one of said heating electrodes,

a plurality of variable capacitors connecting the other of said heating electrodes to said conductive shield,

a variable capacitor connecting the other of said heating electrodes to a location on one of said conductors that is close to the short circuited interconnection between said conductors,

and an electrical connection between the short circuited interconnection between said conductors and said conductive shield.

6. A dielectric heating system for heating a dielectric Workload disposed between heating electrodes comprising a source of radio frequency energy,

a high Q, low impedance tank circuit comprising two conductors interconnected at one end by a short circuit and at the other end by a capacitor,

capacitive means coupling said source of radio frequency energy to said tank circuit,

resonant means also connecting said source of radio frequency energy to said tank circuit, and means connecting said tank circuit to said heating electrodes for transferring energy to said electrodes. 7. A dielectric heating syhtem for heating a dielectric workload disposed between heating electrodes comprising:

a source of radio frequency energy; a high Q, low impedance tank circuit comprising an inner cylindrical conductor,

an outer cylindrical conductor,

an annular conductor interconnecting said cylindrical conductors at one end and supporting said cylindrical conductors in a coaxial relationship, and

capacitive means interconnecting said conductors at the other end;

first and second reactive circuit means connecting said source to the other end of said conductors;

third reactive circuit means connecting said inner cylindrical conductor to one heating electrode; and fourth reactive circuit means connecting said outer cylindrical conductor to the other heating electrode.

8. A system for heating a dielectric workload disposed between heating electrodes comprising:

a conductive shield surrounding said heating electrodes;

circuit means electrically connecting one of said heating electrodes to a first wall of said conductive shield;

variable capacitor means comprising a plurality of variable capacitors each of which connects the other of said heating electrodes to said second wall of said conductive shield to form a radio frequency energy storage circuit; and

energy transfer means connecting said source of radio frequency energy to said energy storage circuit for transferring energy from said source into said energy storage circuit.

9. The dielectric heating system set vforth in claim 8 in which the plurality of variable capacitors comprising the variable capacity means are spaced far enough from one another to form a plurality of inductively coupled circuits which can be individually controlled by adjustment of said variable capacitors.

10. A system for heating a dielectric work load disposed between heating electrodes comprising:

. a low impedance source of radio frequency energy;

a conductive shield surrounding said heating electrodes;

circuit means electrically connecting one of said heating electrodes to the first wall of said conductive shield;

a series circuit comprising variable capacitor means connected in series with said low impedance source of radio frequency energy, said series circuit connecting the other set of heating electrodes to a second Wall of said conductive shield; and

additional variable capacitor means connecting the other set of heating electrodes to said second wall of said conductive shield.

11. The dielectric heating system of claim 10* including first adjustment means associated with the variable capacitor means included in said series circuit for controlling the amount of energy that reaches the heating electrodes,

and additional adjustment means associated with said additional variable capacitor means for controlling the distribution of energy between said heating electrodes.

References Cited UNITED STATES PATENTS JOSEPH V. TRUHE, Primary Examiner L. H. BENDER, Assistant Examiner U.S. Cl. X.R. 

